Violence toward infants by adult males is thought to occur in many species, but relatively little is known about the factors which influence the behavior of adult males toward young. The experiments contained in this proposal concern the regulation of infanticide (the killing of young) and parental behavior in male house mice (Mus musculus). I recently found that experience mating markedly influences the behavior of male mice toward young: between 12 and 50 days after mating most males behave parentally toward young, while before and after this time most males commit infanticide. The striking feature of this finding is that a male's own young would be born 19 days after mating and then normally weaned bout 50 days after mating. Thus, male mice are inhibited from committing infancticide after mating exactly during the time that they could be in contact with their own offspring. Previous experiments have revealed that mating per se, rather than recognition of an individual female and her young, triggers these changes in behavior. The first set of proposed experiments will provide information concerning the stimuli associated with mating and the sensory pathways involved in mediating these changes in behavior. In addition, the brain areas involved in mediating infanticide and parental behavior will be investigated. Finally, experiments to determine whether pituitary and/or gonadal hormones influence changes in behavior toward young after mating will be conducted. The second set of experiments will involve examining male mice that develop in utero between two male fetuses (2M males), between a male and a female fetus (1M males), and between two female fetuses (OM males). OM male fetuses are exposed to high titers of estradiol during fetal life relative to 2M male fetuses, and most sexually-naive adult OM males commit infanticide while most sexually-naive adult 2M males behave parentally toward young (1M males are intermediate in behavior between OM and 2M males). OM, 1M and 2M male mice will be delivered by Cesarean section and examined in adulthood for their behavior toward young both before and after mating. This experiment will thus assess whether prior intrauterine position limits the degree to which adult experience can influence behavior toward young in male mice.